(1) Field of the Invention
The present invention relates to an image forming apparatus and a power transmission mechanism that is suitably used in the image forming apparatus.
(2) Description of the Related Art
An image forming apparatus such as a copier, a printer, a fax machine, or a complex machine of these machines has a typical structure where a recording sheet is fed from a paper feeder and transported along a transport path, a toner image corresponding to image data is transferred onto the recording sheet, and the toner image on the recording sheet is fixed onto the recording sheet by a fixing unit.
In general, the recording sheet is transported through the transport path by a sheet transport device using a pair of rotational members such as rollers and belts. In the sheet transport device, for example, a nip is formed between a pair of rollers that are pressed against each other. One of the rollers is connected, via a power transmission system, to a motor as a drive source. One of the rollers is driven to rotate by a power transmitted from the motor, then the other roller rotates following the rotation of the rotating roller.
In the nip, the recording sheet is transported in the state of being clamped between the pair of rotating rollers.
In such a sheet transport device, if the transported recording sheet is stuck in the nip and a paper jam occurs, it is necessary to remove the recording sheet from the nip by pulling off the sheet. In that case, since one of the pair of rollers is connected to the motor as a drive source, to pull off the recording sheet, a force larger than the torque of the roller is necessary.
However, when the force that pulls the recording sheet is larger than the strength of the recording sheet, the recording sheet may not be pulled from the nip and may be torn apart. Fragments of the recording sheet then remain stuck in the nip. If a small fragment of the recording sheet is stuck in the nip, it is difficult to remove the fragment from the nip.
Also, in the fixing unit, while the recording sheet passes through the fixing nip formed by a pair of rotational members, the recording sheet is heated and pressed so that the toner image thereon is fixed onto the recording sheet. In such a fixing unit, to fix the toner image onto the recording sheet, it is necessary to apply a high pressure to the recording sheet in the fixing nip. Thus, if a recording sheet is stuck in the fixing nip and a paper jam occurs, a large pulling force is necessary to pull off the recording sheet from the fixing nip. This increases the possibility that the recording sheet is torn apart.
Patent Literature 1 (Japanese Patent Application Publication No. 2006-214567) discloses a structure in which a coupling, which is provided in a drive transmission system (power transmission mechanism) for transmitting a drive force to a roller to transport a recording sheet (sheet), is caused to get into a power non-transmission state by a connection release member (connection release lever). The coupling is provided in a slidable manner and gets into a power transmission state when it is caused to engage with an idle gear by an urging force from a coil spring. The connection release lever moves in conjunction with the opening/closing operation of the opening/closing door, wherein the sheet transport path is exposed to outside when the opening/closing door is open. When the opening/closing door is open, the coupling is caused to slide resisting the urging force from the coil spring, and the coupling comes out of the engaging state with the idle gear and gets into the power non-transmission state.
According to the structure of Patent Literature 1, the power transmission state is generated by causing the coupling to engage with the idle gear by the urging force from the coil spring. Thus, a large frictional force is applied to between the coupling and the idle gear, and the coupling or the idle gear is likely to wear with time.
According to the structure of Patent Literature 1, to release the engagement between the coupling and the idle gear, a relatively small power is used to move the connection release lever. However, even with this structure, when an attempt is made to release the engagement between the coupling and the idle gear, both the urging force from the coil spring and the pressure from the connection release lever are applied to the coupling. This may slant or distort the slidable coupling relative to the drive shaft.
When it happens, the coupling becomes difficult to slide smoothly relative to the drive shaft. Also, since the coupling receives a large pressure, the coupling may be deformed. In that case, the coupling may not be able to engage with the idle gear smoothly, and switching between transmission and non-transmission of power may not be conducted stably for a long time period in the power transmission system.